A number of batteries in wide-spread use in the U.S. Armed Forces and industry do not exhibit a voltage drop prior to discharge to a useless level. Instead they exhibit constant voltage until nearly all the charge in the battery is exhausted; then, the voltage drops rapidly and no further power is produced. The life of such a battery cannot be determined by conventional techniques such as voltage measurements. Lithium batteries such as those used in man-pack radios are an example of such a battery. Because the life or failure point of such batteries cannot be easily determined, soldiers will often replace batteries with significant lifetimes remaining rather than chance their failure in the field. This wastes batteries causing the Army to purchase large numbers of excess replacements. These discarded batteries are considered hazardous waste because they have not been fully discharged.
A given battery design provides a fixed number of coulombs during its useful lifetime; therefore, batteries which exhibit this type of behavior could be tested if a circuit could be attached which would determine the number of coulombs passed. Known electronic circuits perform this task either capacitively or digitally. The former has a tendency to leak the charge stored causing inaccurate readings over long periods of time, while the latter is cost prohibitive.
In other applications unrelated to batteries specifically, but generally related to electrical systems where an indication of actual total time of operation is required, coulomb measuring devices may be useful. For example, the running time of an aircraft engine, truck engine, rental car engine, or other electrically related devices may be of interest for maintenance scheduling, and so forth.
To date, no cost effective device has been produced which could provide a coulometric monitoring of a battery, power source, or other current source. Such a device could provide an indication of a certain coulomb limit, a series of coulomb limits, or continuous status of coulombs passed in constant voltage batteries such as lithium batteries.